This invention relates to countermeasure techniques for protecting aircraft from enemy attack and, more particularly, to a means of protecting an aircraft by deceiving an enemy into tracking a decoy target.
Various systems are being employed and have been proposed for protecting aircraft from enemy attack, including the use of radar jamming equipment on the aircraft. When the threat is from tracking or missile seeker radars employing monopulse or pseudo-monopulse angular error sensing mechanisms, the difficulty of countermeasuring increases since few effective electronic countermeasure techniques against them exist. One of the well-known techniques is that of producing false targets at a physical distance from the aircraft under attack.
One means of producing such false targets is to employ expendable, active decoys. These devices are ejected from an aircraft when under attack and usually contain active electronic components for transmitting signals which confuse and deceive an enemy radar or missile into tracking the decoy rather than the aircraft itself.
One type of such expendable, active decoy includes a single port, keyed primed oscillator which is primed with the low level signal received at its antenna from an enemy""s tracking radar. The priming signal is injected directly into the tank circuit of the oscillator which is being rapidly turned on and off (keyed), producing a rapidly pulsed signal which tracks the frequency of the priming signal. The resultant signal from the oscillator is transmitted back to the receiving section of the enemy radar, keyed at such a high PRF that the receiver cannot distinguish the individual pulses of returned energy. Thus, the signal from the decoy is detected as a replica of the original radar transmission, even if it is a CW signal. This type of decoy has worked very well, but is limited in its ability to make full use of the average power capability of the components used for transmission. This limitation is due to the fact that the primed oscillator is keyed such that it has a duty factor very much less than 100% and, accordingly, the average power which can be transmitted from components which are peak power limited is restricted.
An alternative approach for expendable, active decoys is to employ a two-port device, having both an input and an output antenna wherein a signal from the enemy radar, which is received at a first antenna, is amplified in the expendable decoy, and then transmitted back to the radar from a second antenna. While this technique apparently permits generation of larger average power from the components than using the keyed, primed oscillator, it also has severe gain limitations since it is extremely difficult to isolate the receiving and transmitting antennas adequately from each other, particularly in an expendable device of relatively small size. If such a device is operated at a high duty factor, the coupling from the transmitter to the receiver would cause interfering feedback. Therefore, it is required that such a device be operated at reduced duty factor which reduces the effective power output and, thus, at a corresponding reduction in effectiveness.
Accordingly, it is an object of this invention to provide improved electronic countermeasures.
It is another object of this invention to provide improved expendable decoy countermeasures.
It is a further object of this invention to provide improved electronic countermeasures in which the electronics is, in part, located in the aircraft to be protected and in part, in expendable decoys.
Briefly, in one embodiment of the invention, improved electronic countermeasures is provided by greatly increasing the separation between the decoy""s receiving and transmitting antennas by placing the radar signal sensing receiver and its antenna on the aircraft and the countermeasures transmitter and its antenna on the expendable decoy. This provides ample isolation between the transmitter and receiver such that the transmitter can be operated continuously at maximum effectiveness. The incoming signal from, for example, an enemy tracking radar is applied to a receiver in the aircraft to be protected and mixed with a signal generated by a local oscillator in the receiver. The signal from the mixer, at a frequency which is the difference between that of the received radar signal and the local oscillator, is transmitted to the expendable decoy along with the signal from the local oscillator as a reference. As the decoy is only a short distance from the aircraft and trailing behind, a rearward facing antenna and very low power transmissions can be used by the aircraft, minimizing the probability of detection of its transmissions by the enemy.
These two signals are received from the aircraft at the decoy, amplified and mixed to generate a signal at a frequency which is the sum of that of the local oscillator and difference signal and, thus, at the same frequency as that of the tracking radar. The signal is then transmitted by the decoy to the tracking radar. The signal is a duplicate of the original radar signal and due to the nature of the decoy trailing behind and falling away from the aircraft, the enemy radar is given false bearing, range and doppler information.
The signal transmitted by the expendable decoy is at a completely different frequency band from that of the two signals it receives from the aircraft. Therefore, both the transmitter and receiver in the expendable decoy can be made to share a common antenna without any problem of mutual interference.